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Thursday, October 30, 2014

I have been working on interfacing the PIC32MX250F128B with a small 2.2" TFT display from Adafruit. It's a nice little display that is fairly easy to communicate with, using SPI communication. The display I'm using is: http://www.adafruit.com/product/1480

Adafruit provides nice open-source libraries for their products. However, they are for Arduino and thus cannot be directly reused for the PIC32. I went through the library and ported it over for the PIC32, in C. I have attached my project file as a .zip file and you can download it to go through the library header and source files, as well as the demo code. I've tried heavily commenting the code so that it is self-explanatory.

As far as hardware goes, with the demo code, the pin connections for the display are:

BL (backlight): I left it unconnected, but you can connect it to 3.3V for backlight.
SCK: connected to RB14 on the PIC
MISO: left unconnected, since I'm not reading anything from the screen
MOSI: connected to RB11 on the PIC
CS: connected to RB1 on the PIC
SDCS: left unconnected as I'm not using the microSD card for this
RST: connected to RB2 on the PIC
D/C: connected to RB0 on the PIC
VIN: connected to 3.3V supply
GND: connected to gnd

The pins I used are defined in the code and you can easily change them as required.

So, the simple way of offloading the SPI update to the DMA module would be to let the DMA channel transfer data to the SPI buffer. The SPI module is configured for 16-bit data transfer (since the MCP4822 write command requires 2 bytes). This means that the cell size for the DMA channel has been set to 2 (2 bytes to transfer once triggered). The DMA transfer is triggered by a timer interrupt. Unlike the previous example where I used Timer 1 and its interrupt for the SPI transfer, here I use Timer 2. The reason for this is that, if I want the entire process to be offloaded from the CPU, the SS (Slave Select) or CS (Chip Select) also has to be done entirely in hardware. For that I used the Output Compare 1 module. To use the OC1 module, either Timer 2 or Timer 3 (or the combined 32-bit timer) has to be used. So, I just went with Timer 2.

Configuring the DMA module for transferring data to the SPI buffer was the simple part. I found the configuration of the SS/CS pin the more challenging part. The idea here was to use the OC module in "dual compare mode continuous output pulses" mode. The OC module in this mode generates continuous pulses - the output pin OC1 - which I used as CS/SS - is set high one PBCLK (peripheral bus clock) after the Timer value reaches OC1R; the OC1 pin is cleared one PBCLK after the Timer value reaches OC1RS. Since SS/CS is active low, I set ( (period register) - 4) to be OC1RS and a variable CSlength to be OC1R. CSlength was chosen to be 80% of the period register. What this meant was that. One PBCLK after the Timer reached the (period register - 4), the OC1 pin went low (CS/SS went low) "selecting" the DAC. After about 0.80*(period register) from there, the OC1 pin went high. This means that the CS pin is low for 80% of the period - it goes low a small time right before the DMA transfer happens and is raised high late enough, after the DMA transfer occurs. I determined that 80% the period was enough time since that is higher than the time required to shift out 16 bits of data at 20MHz SPI clock. See Fig. 2 below for an illustration of the operation of the output compare module in the "dual compare mode continuous output pulses" mode:

Beyond that, the idea is simple. There is a sine table that is the DMA source. The key thing to remember here is that the 12-bit data must be OR-ed with 0x3000 since that is required for the DAC (to set gain=1, shutdown = 0 and channel = A):

Since the entries in the sine table are of "short" data type (each element occupies two bytes), the source size (in bytes) is twice the number of elements. The destination source size is two bytes since I'm transferring two bytes to the SPI buffer register. The cell size is two bytes since the DMA channel has to transfer two bytes (16 bits) at a time to the SPI buffer register. The DMA configuration is:

A point of note is that, the DmaChnSetTxfer( ) takes in the virtual addresses of the source and destination, not the physical addresses. The virtual-to-physical address conversion is done by the function itself as opposed to it being required manually when doing register level operations (as done in my previous DMA example article).

To demonstrate that the DAC control is done with no CPU overhead, in the main( ) function, I have an infinite loop that is just toggling RA0. The output is checked on an oscilloscope:

Fig. 3 - RA0 toggle being demonstrated

Observe that the frequency of the square wave is 4.0MHz - the same that would be observed if all the PIC was doing was the pin toggling.

This also gave better performance than my previous experiment where I did not use DMA. The output sine wave has a higher frequency than observed before and this matches exactly as expected: 400kHz/32 = 12.5kHz as seen (see Fig. 1).

The rest should be very easy to understand and self-explanatory. If you have any questions or suggestions, feel free to comment!

Saturday, October 11, 2014

I found them to be neat little devices. In a small 8-pin PDIP package, the MCP4822 comes with 2 12-bit DACs, which you can easily configure over SPI. This was a great opportunity to get a simple PIC32 SPI application going. I worked on this today to see how fast I can get the DAC output going.

Here's the pinout of the MCP4822:

Fig. 1 - MCP4822 pinout (taken from datasheet)

The MCP4822 can be supplied a voltage in the range of 2.7V to 5.5V. Since I use 3.3V for my PIC32, I used the same 3.3V supply for the VDD for the MCP4822. Pin 5 is the active low signal LDAC that is used to synchronize the two DAC channels. When this pin is brought low, the data in the DAC's input register is copied to the output and both outputs are updated at the same time. I just had this tied to ground. VoutA and VoutB are the two output pins. The other pins are the regular pins for SPI communication - CS (active low) chip select, SCK serial clk, SDI serial data in.

The datasheet provides a nice diagram explaining the timing and pin functions very nicely:

Fig. 2 - Write command for MCP4822 (taken from datasheet)

The write command to the MCP4822 is a 16-bit wide command sent over SPI. Bit 15 (A/B) selects which channel you're sending data to: 1 signifies channel B, 0 signifies channel A. Bit 13 (GA) selects the gain. The MCP4822 has an internal 2.048V reference it uses for the DAC. So, that means that with a gain of 1, the maximum possible output voltage is 2.04975V (4095/4096 * Vref). If a higher output is required, the gain can be increased. The MCP4822 has a configurable gain of 1 or 2. When GA = 1, gain = 1; when GA = 0, gain = 2. Bit 12 SHDN is the shutdown signal. When SHDN is low, the output of the DAC is shut down. The 12 data bits from there on: D11 to D0 ([D11:D0], [bit11:bit0]) are the 12 data bits for the digital to analog conversion.

While I typically use the registers and configure them manually, I decided to use the Microchip plib (peripheral library) here for the SPI module. The peripheral library consists of a lot of low-level functions and macros that essentially require understanding the peripheral but just make the code nicer and easier to read.

I decided to clock the MCP4822 at 20MHz (max for MCP4822) to get quick data transfers.

The SPI module can be initialized using one of 2 options:
1) Using the OpenSPIx(config1, config2) and SpiChnSetBrg(spi_channel, spi_brg) functions
2) Using the SpiChnOpen(spi_channel, config, spi_divider) function

The code block above has code written using both the initialization functions (one is commented out). Essentially, the configuration is that SPI channel 2 is enabled as a master, it is configured for 16-bit data transmission, the SDI pin is disabled, it is configured for serial output change from active high (1) to active low (0) as required by the MCP4822 (see Fig. 2) and the clock divisor is set to 2 so that SPI clock = 20MHz.

The SCK pins for the PIC32 are mapped physically to the RB15 (SCK2) and RB14 (SCK1) pins (check pinout on page 4 in datasheet). The SDO and SDI pins are remappable pins and thus require you to map them to physical pins. The SDI pin here is unused since the PIC32-to-MCP4822 is a one way communication with the PIC32 as the master and the MCP4822 as the slave.

The datasheet provides the table for all the PPS (peripheral pin select) mappings. The relevant section for the SDO is:

Table 1 - PPS configuration for SDO

I decided to use RPB5:

PPSOutput(2, RPB5, SDO2); // map SDO2 to RB5

The connections I used for the MCP4822 are:

Fig. 3 - MCP4822 connections

The chip select / slave select line (CS as shown on the MCP4822 pinout, see Fig. 1) has to be controlled manually. (For a hardware-based processor-free control scheme, see PIC32 DMA+SPI+DAC : Analog output synthesis with zero CPU overhead.) This is an active low chip select signal. When data is being sent to the MCP4822, while the SPI transaction is occurring, the CS pin must be kept low. Initially I checked to see if there was a way the hardware would take care of that. I looked into the framed SPI mode. However, this did not serve my purpose. From what I've read, the framed SPI mode is an SPI mode where the clock is always output (instead of only during transactions as traditionally done). However, the hardware generates a low sync signal (by pulling CS low) before the data is transmitted from the PIC32. The problem here was that the CS pin was pulled low and kept low for one SPI clock period before raising CS high as data is transmitted out from SDO. This isn't going to work for the MCP4822 since the chip requires that CS be held low during the entirety of data transmission (see Fig. 2). So I just decided to use RA4 as the pin for CS. Before starting any transmission, I pull CS low in software and raise it high at the end.

Since I called this function from the ISR in my code, I decided to make it an inline function to eliminate function call overhead.

In my test code, I just wanted to see how fast an output I can get. So I clocked the MCP4822 at 20MHz and tried to maximize the frequency of the output signal I generate: I used channel A to generate a sine wave output and channel B to generate a triangle wave output. I used a timer (Timer 1) to control the timing and in the ISR, I just called the writeDAC inline function to generate outputs as required from two pre-generated tables.

You can clearly see the SPI clock during transmission here. From the measurement panel on the right you can see that the SPI clock is 20MHz. There are also 16 pulses you can count - since a write command to the DAC consists of 16 bit transfers. CH2, on the bottom, shows the data being transmitted. Since the first signal (leftmost) is a 1, and since this corresponds to bit 15, this is a snapshot of data being transmitted to DAC channel B.

While this was a quick test I did to test the PIC32 SPI and the MCP4822, this is in no means a complete evaluation of the MCP4822. For example, I updated the DAC more quickly than I should have, to push for speed, as this didn't give enough of a settling time. However, I was satisfied with the results and so, stuck with that. I will do some further testing on this later.

For more details of the PIC32MX250F128B SPI module, refer to the datasheet and reference manual. For more details on the MCP4822, refer to its datasheet.

If you have any questions, feel free to ask. Let me know what you think, in the comments section below!

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About Me

I am Syed Tahmid Mahbub, from Dhaka, Bangladesh, born on August 1, 1994.
Electronics is my passion and from class V, I have been learning electronics. I learnt and worked mostly on SMPS, power electronics, microcontrollers and integration of microcontrollers with SMPS and power electronics. I've used PIC and AVR microcontrollers - PIC 10F, 12F, 16F, 18F, 24F, dsPIC 30F, 33F, PIC32, ATmega and ATtiny, integrating them with various SMPS and power electronics circuits.
I have completed my Bachelor's degree from Cornell University (Class of 2017) in Ithaca, New York, USA, majoring in Electrical and Computer Engineering (ECE).
I am a member of the forum www.edaboard.com, where I am an "Advanced Member Level 5" (the highest level attainable) and also the forum allaboutcircuits.com, where I am a "Senior Member". I post to help solve electronics-related problems of engineers and engineering students from all over the world.
I love watching and playing cricket and football (soccer), and listening to music.
I am now a hardware engineer at Apple in Silicon Valley, California, USA.